Method and apparatus for melt-spinning hollow fibers

ABSTRACT

A process and apparatus are disclosed for melt-spinning hollow fibers under precisely controlled pressure conditions to provide more uniform properties for reverse-osmosis separation.

United States Patent [151 3,686,377 y [451 Aug. 22, 1972 [54] METHOD AND APPARATUS FOR [56] References Cited MELT-SPINNING HOLLOW FIBERS UNITED STATES PATENTS [72] Inventor: Richard Young Hays, Kinston, NC.

2,546,629 3/1951 Brillhan ..264/209 1 1 9 du P911149 Nemours and 2,760,228 8/1956 Verges ..264/209 pany, Wilmington, Dcl. 3,092,874 6/1963 Fallwell ..264/40 3,331,901 7/1967 Thomas ..264/209 I 3,397,427 8/1968 Burke et a1 ..18/8 sc [21] Appl. No.: 119,744 3,421,873 1/1969 Burgman et a1. ..264/167 3 Primary Examiner-Jay H. W00 [52] U.S. C1 ..264/40, 18/8 SC, 264/94, AHOmey NOrriS Ruckman 264/99, 264/177 F, 264/209 [51] Int. Cl. ..B29c 17/06, B29d 23/04 [57] ABSTRACT [58] Field of Search ..18/8 SC; 264/ 177 F, 40, 94, A process and apparatus are disclosed for mew 264/991 209; 65/1 spinning hollow fibers under precisely controlled presv sure conditions to provide more uniform properties for reverse-osmosis separation.

2 Clains, 2 Drawing Figures PATENTEI] M1222 m2 FIG- R O N E V m RICHARD YOUNG HAYS BY M ATTORNEY METHOD AND APPARATUS FOR MELT-SPINNING HOLLOW FIBERS This invention concerns the spinning of hollow fibers. More specifically, it concerns the spinning of hollow fibers useful for reverse-osmotic separation of components from gas and liquid streams through the permeable fiber wall. Further,it concerns an improved process and apparatus for spinning these fibers.

Hollow fibers have become increasingly useful in industrial applications where permeability of the fiber wall effects separation of multi-stream components, e.g., brackish water can be made potable and multicomponent gas streams can be selectively separated.

, U.S. Pats. Nos. 3,228,877 and 3,524,546 disclose usefulness of these hollow fibers when assembled into a bundle to which process streams can be supplied. A high pressure differential is applied across the tube wall to diffuse components in the opposite direction to that if no pressure differential existed. Increasing the pressure differential, or decreasing the wall thickness, increases the rate of separation. Hence, a hollow fiber having a uniform wall thickness and outside diameter with a high burst-strength is needed.

I-Iollow fibers can be spun using a post-coalescing spinneret such as shown in US. Pat. No. 3,313,000 but are usually spun using a vented-insert spinneret such as shown in US. Pat. No. 3,397,427. Here an insert is set into each hole in the spinneret, the insert providing an annular flow-channel for the polymer to be spun and providing a centrally located gas-passage connecting to the outside of the spinneret. Melt from the polymer system is forced through the annular passage while gas flows through the central passage into the hollow space at the center of the filament. Gas may be aspirated into the center due to vacuum generated by melt flowing in the annular space or, alternatively, may be supplied at pressure to the central passage in the spinneret insert. This is known to affect the percent hollow of the fiber as disclosed in US. Pat. No. 3,014,237. Percent hollow is defined as:

(inside diameter) Uniformity of wall thickness, percent hollow, outside diameter and burst-strength of the fibers become especially critical in reverse osmosis applications since these affect accurate prediction of permeation rates, the number of functioning tubes which can be housed in processing equipment, and operating pressures to which these separation devices can be subjected.

Many variables in the spinning process affect the dimensions of hollow fibers. For example, as the spinning-pack ages, it accumulates debris, gel, etc., which increase the pressure drop across the pack. Since the metering pumps deliver at constant volume, the increased pressure-drop causes increased shear workinput to the flowing polymer-stream. Polymer then exits from the spinning-pack at higher temperature and thus reduced viscosity. This reduced-viscosity melt supplied to the spinneret results in reduced percent hollow fiber. Pack pressure-drop does not increase at a constant rate and the resulting spun-viscosity cannot be adequately compensated by changes in melt-temperature since spinning-machine temperatures cannot be quickly or precisely changed.

OBJECTS OF THE INVENTION It is an object of this invention to provide a process and apparatus for making hollow fibers having more uniform properties and increased burst strength, making them more suitable for reverse-osmosis separation than those previously attained. It is further an object to provide a process and apparatus for making hollow fibers which, by a simple and economical means, can precisely compensate for variations in spinning conditions, such as viscosity and temperature.

SUMMARY OF THE INVENTION In accordance with the present invention molten polymer is extruded in the form of thin-walled hollow filaments from spinneret orifices having an annular melt-passageway surrounding a central air channel supplied with air at low pressure to provide a hollow in the filament. The air pressure is controlled relative to pressure outside the filament by venting the air supply from beneath liquid in an open container, to limit the pressure, and passing the air beneath liquid in a closed container to reduce the air pressure to a value controlled by the differential depths of liquid in the two containers. The air is supplied from the closed container to the central air channel at a highly uniform pressure differential relative to atmospheric pressure around the filament to maintain a uniform hollow in the filament. Because of the extremely fine control of pressure, and ease with which the pressure can be adjusted to compensate for changes in polymer flow, the filaments produced have outstandingly uniform properties with increased burst-strength.

In the apparatus for practicing this process, gas from a suitable source is supplied at a controlled rate through a tube connected to the side arm of a T. One leg of the T is connected to a dip tube immersed in liquid in an open-top container. The other leg of the T is connected to a dip tube immersed in liquid in a closed container. A tube leading from the top of the closed container conducts gas, at a pressure determined by the relative depths of immersion of the two dip tubes, to the axial bore of a spinneret of the type disclosed in US. Pat. No. 3,397,427. One or both of the dip tubes is adjustable to regulate the gas pressure by changing the depth of immersion.

Spinning by the above process permits rapid and precise adjustment of gas supply-pressure to compensate for changes in melt-viscosity. Further, spinning by this process with spinneret capillary and hollow insert sized to produce a thicker walled filament exiting the spinneret-face, while increasing the gas supply-pressure, increases fiber burst-strength.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic representation of the differential pressure-control apparatus of this invention.

FIG. 2 shows an axial cross-section through a singlevented spinneret insert for spinning hollow fibers using the differential-pressure-controlling means of this invention.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 shows a source of gas such as a nitrogen cylinder 27 with a conventional pressure-regulator 29 connected to tubing 31 and hand-adjustable throttlingvalve 33. Valve 33 connects to T 45, which connection enters the dead-end side of the T. The straight side of the T is connected on one side to shut-off valve 35 which connects to direct-acting dip-tube 47, immersed in any suitable fluid 37 in open-top container 38. The other side of the T is connected to reverse-acting dip tube 39 in fluid filled closed container 41 to reduce the gas pressure. Line 43 connects from the closed container to the spinneret assembly shown in FIG. 2. The orientation shown for T 45 is critical to obtain accurate control, since wide pressure-fluctuations will result if the T is connected straight through from the regulator to the second dip-tube assembly. Depth of immersion of dip-tube 47 in the liquid in container 38 determines the pressure at which gas will be supplied to the second dip-tube 39..Any excess gas supplied through dip-tube 47 simply vents through open container 38. The difference in depth of immersion between dip-tube 39 and dip-tube 47 determines the pressure at which gas will be supplied through conduit 43 to the spinneret. Precise control of the pressure differential relative to atmospheric pressure around the spinneret assembly is therefore maintained. 7

FIG. 2 shows one portion of a spinneret-plate l sealed against spinning-block 2 by gasket 3. Spinneret melt-passageway 7 has an insert 5, such as described in US. Pat. No. 3,397,427, which channels liquid polymer into annular melt-passageway 9. Gas from conduit 43 (FIG. 1) enters heater-passage 11, which is equipped with jacket 13 for a heating-fluid such as Dowtherm. Gas temperature is increased to approxi- 4 ample H is 1.28 times the ratio of the viscosity of a solution of 0.80 gms. polymer in 10 ml. of hexafluoroisopropanol (HFIP) reagent grade, to the viscosity of I-IFIP, both at 25 C.

EXAMPLE I This example shows the uniform properties of hollow fibers spun by the process of this invention. Table I shows the results of two spin tests. Nylon 66 of nominally 70 RV is supplied to the spinning-machine, spun at the same melt feed-rate and wound up at the same speed in both tests. Spin No. 1 uses the vented-insert spinneret of US. Pat. No. 3,397,427 and aspirates spinning-room air into the central vent due to pumping action of melt flowing through the annular space. Spin No. 2 uses the same spinneret-insert and capillary size as Spin No. l but Spin No. 2 employs the process shown in FIGS. 1 and 2 to maintain a positive gas-pressure to the vent. Spinning-block temperature during Spin No. 1 is varied between 268 and 285 C., endeavoring to fix percent-hollow at 32 i 2, whereas spinning-block temperature for Spin No. 2 is set at 300 C. and maintained between 296 and 302 C. Based on 900 psi. minimum burst-pressure and 30 to 34 percent hollow specification, desired fiber-yield increases approximately 30 percent using this differential-pressure spinning-process. Further, the range of percent-hollow and burst-strength is narrowed significantly to produce much more uniform hollow fibers. The differential pressure is varied between 0.1 to 0.20 inch water in TABLE I.HOLLOW FIBER SPUN WITH AND WITHOUT DIFFERENTIAL PRESSURE TO SPINNERE'IINSERT Percent fibers produced at Percent fibers produced at each percent hollow each burst strength Percent hollow. b Burst strength (100 p.s.i.g.).

mately polymer-temperature at the heater exit. Heated gas passes through channel 12 in the spinneret-body and enters annular chamber 14 extending around the spinneret periphery. Gas from annular chamber 14 enters passage 15 and thence channel 17 positioned centrally within melt-passageway 9. Only one spinneretorifice is shown of a typical multi-orificed spinneret. Gas is supplied to all orifices from annular chamber 14. Spinneret-insert 5 terminates coplanar with the lower face 19 of spinneret-plate 1. Gas supplied through channel 17 into the filament gas-channel 25 created by insert 5 maintains a uniform hollow in the filament. The hollow filament exiting from the spinneret displays the characteristic bulge 21 and thereafter begins to neck down in diameter. The outside diameter of the insert 5, through which gas enters the hollow filament, is shown by A and the inside diameter of the capillary or annular melt-passage is shown by B.

In the following examples, which illustrate the process of this invention, RV (relative viscosity) of the nylon polymer of Example I is the ratio of the viscosity of a solution of 5.5 gms. of nylon polymer in 50 mls. of 90 percent formic acid to the viscosity of 90 percent formic acid, both at 25 C.; RV of the polyester of Ex- Spin No. 2 to maintain desired hollow-fiber properties during the spin.

EXAMPLE n This example shows the increased burst-pressure of hollow fiber obtained with the differential-pressure spinning-process of this invention. A polyester fiber is spun under identical melt and feed conditions in Spin Nos. 3, 4 and 5. Results are shown in Table II. Nominal polymer relative viscosity is 52. Spin Nos. 4 and 5 use the differential-pressure spinning-process of FIGS. 1 and 2. The spinneret of US. Pat. No. 3,397,427 is used except the increased wall-thickness exiting from the spinneret is attained by increasing the inside diameter of the spinneret capillary without changing the outside diameter of the insert. As the fiber wall thickness is increased, the differential pressure supplied to the center of the hollow filament is increased to obtain the same final wound-up fiber-dimensions. The increased burststrength resulting provides a superior fiber for permeation-separation apparatus. Note that the fiber produced in Spin No. 3 with gas aspirating from the spinning area has more polymer per unit length, i.e., a higher denier per filament. The smaller percent hollow indicates a TABLE II.INCREASED fibers, a spinneret comprising a plate having upper and lower surfaces connected a passage, said passage BURST STRENGTH HOLLOW FIBER WITH DIFFERENTIAL PRES- SURE TO VENTED SPINNEREI INSERT Fiber spinneret dimensions, Mils Windup dimensions Diifer- Fiber speed, microns ential burst Demer/ yards] Percent Capillary I sort pressure, pressure, Spin No filament minute hollow 0.D. LD. I.D. D. in. H2O p.s.i.g.

l Dimension B, Fig. 2. 2 Dimension A, Fig. 2. 3 Vented to atmosphere.

-It should be noted that this differential-pressure spinning-method also provides means to control the pressure (negative or positive) under which hollow fibers are spun, thus reducing or increasing the percent hollow in comparison with that which could be obtained with a given spinneret aspirating air from the spinning room. The spinneret life is also doubled since the pressure-control means can be isolated and higher pressure gas supplied to the vented spinneret-insert during wiping to prevent plugging the insert with degraded polymer. Hollow fiber definition is thus maintained for longer operating periods.

I claim:

1. In a process for melt-spinning hollow nylon or polyester fibers, the improvement for maintaining a uniform hollow which comprises extruding the molten polymer in the form of thin-walled hollow filaments from spinneret orifices having an annular meltpassageway surrounding a central gas channel, and feeding nitrogen into the filament hollow through the gas channel at a controlled pressure relative to pressure outside the filament by venting the nitrogen supply from beneath liquid in an open container to limit the pressure and passing the nitrogen beneath liquid in a closed container, connected to said gas channel, to reduce the nitrogen pressure to a value controlled by the differential depths of liquid in the two containers.

2. In an apparatus for the production of hollow,

ally spaced holes in communication with said bore andthe tapered length of the passage through the plate, thereby providing a flow path through the spinneret, an axial bore through said second cylindrical portion to supply gas to the interior of a hollow filament formed by extruding molten polymer through said passage; means for supplying gas at a controlled rate to the side arm of a T, a dip tube immersed in liquid in an open-top container and connected to one leg of said T, a closed container, a dip tube immersed in liquid in the closed container and connected to the other leg of said T, and means for conducting gas from the top of said closed container to said axial bore to supply gas to the filament hollow under a uniform pressure determined by the relative depths of immersion of said two dip tubes. 

2. In an apparatus for the production of hollow fibers, a spinneret comprising a plate having upper and lower surfaces connected by a passage, said passage having successive cylindrical, tapered and capillary lengths; an insert including first and second cylindrical portions, said first cylindrical portion having a bottom surface and being swaged into said cylindrical length, said upper surface adjacent said passage being formed into a lip engaging said first cylindrical portion, said second cylindrical portion projecting from said bottom surface substantially through the capillary length and being of smaller cross-sectional dimensions than the capillary length, said first cylindrical portion having an axial bore partially therethrough and a plurality of radially spaced holes in communication with said bore and the tapered length of the passage through the plate, thereby providing a flow path through the spinneret, an axial bore through said second cylindrical portion to supply gas to the interior of a hollow filament formed by extruding molten polymer through said passage; means for supplying gas at a controlled rate to the side arm of a T, a dip tube immersed in liquid in an open-top container and connected to one leg of said T, a closed container, a dip tube immersed in liquid in the closed Container and connected to the other leg of said T, and means for conducting gas from the top of said closed container to said axial bore to supply gas to the filament hollow under a uniform pressure determined by the relative depths of immersion of said two dip tubes. 